Light-emitting diodes (LEDs) are attractive replacement candidates for conventional light sources based on incandescent bulbs and fluorescent light tubes. LEDs have higher energy conversion efficiency than incandescent lights and substantially longer lifetimes than both incandescent and fluorescent light fixtures. In addition, LED-based light fixtures do not require the high voltages associated with fluorescent lights.
Unfortunately, LEDs have a number of disadvantages that inhibit their widespread acceptance as a replacement for the above-described conventional light sources. First, LEDs having outputs equivalent to that of a large conventional light source are not commercially available; hence, high power LED sources require that a large number of individual LEDs be combined to provide the desired output.
Second, LEDs emit light in narrow optical bands. Hence, to provide a light source that a human observer will perceive as having a particular color, LEDs having different emission spectra must be combined into the same light source or phosphor conversion layers must be utilized to convert some of the LED generated light to light of a different spectrum. For example, an LED that is perceived to emit white light can be constructed by combining the output of LEDs having emission spectra in the red, blue, and green region of the spectrum or by utilizing a blue emitting LED and a layer of phosphor that converts some of the output light to light in the yellow region of the spectrum.
The first approach requires three types of LEDs that generate light at essentially the same maximum intensity if the output light is to be perceived as having any color within a wide range of colors. The cost of providing LEDs in certain color bands is substantially higher than that associated with LEDs in other color bands. As a result, the cost of the light source is dominated by the cost of the LED type having the highest cost.
Phosphor conversion can provide a solution to this problem if the light source is to provide a single color of light. For example, as noted above, a white light source can be provided by covering a blue LED with a layer of phosphor that converts some of the blue light to yellow light. If the fraction of the light converted is correctly chosen, a human observer will perceive the light as being white. The cost of such a light source is dominated by the cost of the blue LED. Cost efficient white light sources of this type are now being sold; however, these sources only generate white light of one “color temperature”.
For many applications, the ability to control the color temperature of the light source, or otherwise vary the output color in a narrow range about that provided by the white light source is required. For example, physicians often view x-rays with the aid of a backlit display constructed from a light box that contains a white light source that uniformly illuminates one surface of the box on which the x-ray is placed. Two different color light sources are used in conjunction with these displays, the particular color being determined by the particular type of x-ray being displayed.
Prior art light boxes utilize fluorescent light tubes as the light source. Since fluorescent lights do not provide changeable color points, the user is required to maintain two different viewing light boxes. This arrangement substantially increases the costs of the viewing boxes, which are typically located in a number of different examining rooms in the physician's office. In addition, the second box uses a significant amount of the wall space in the examining room.